Why am I not able to participate in a regatta?
Most likely you are not able to participate because you missed the race start. In other words, you either entered a co-op or checked your Participate in Regatta status after the regatta had begun. In this case you can follow your co-op progress in the regatta, but you can’t pick a task to contribute to it.
According to rules, a group of participating co-ops is formed based on their size. It just won’t be fair to give advantage to any co-op by adding a new member to the race. There’s a one-day break between regattas when you have time to change co-ops.
KEEP IN MIND that if you leave your co-op before the end of the race, you won’t be able to claim rewards.